Question

What are the types of isomerism exhibited by ketones?

Short answer

Answer: Ketones can exhibit two types of isomerism: structural isomerism (chain isomerism and positional isomerism) and stereoisomerism (optical isomerism).

Step-by-step solution

1. Structural Isomerism

Structural isomerism occurs when compounds have the same molecular formula but different structural formulas. For ketones, there are two common types of structural isomerism: chain isomerism and positional isomerism.

1.1. Chain Isomerism

Chain isomerism happens when the carbon chain in the molecule is arranged differently. For example, consider the molecular formula C4H8O, which can represent two different ketones: (a) Butanone (CH3COCH2CH3): A straight-chain ketone with the carbonyl group in the middle. (b) 2-Methylpropan-2-one (CH3COC(CH3)2): A branched-chain ketone with the carbonyl group bonded to a tertiary carbon atom. Both of these ketones have the same molecular formula but different structural formulas, so they are chain isomers of each other.

1.2. Positional Isomerism

Positional isomerism happens when the functional group – in this case, the carbonyl group – is attached to different positions on the same carbon chain. For example, consider the molecular formula C5H10O, which can represent the following ketones: (a) Pentan-2-one (CH3COCH2CH2CH3): The carbonyl group is attached to the second carbon atom in the straight chain. (b) Pentan-3-one (CH3CH2COCH2CH3): The carbonyl group is attached to the third carbon atom in the straight chain. These two ketones have the same molecular formula but different positions of the carbonyl group, so they are positional isomers of each other.

2. Stereoisomerism

Stereoisomerism occurs when compounds have the same structural formula but different spatial arrangements. Ketones can exhibit stereoisomerism only when the carbonyl group is bonded to two different stereogenic carbon atoms. This type of stereoisomerism is called optical isomerism.

2.1. Optical Isomerism

Optical isomerism is exhibited by compounds having chiral centers (stereogenic carbon atoms) in their structure, leading to the formation of non-superimposable mirror images called enantiomers. When the carbonyl group in a ketone is bonded to two different stereogenic carbon atoms, it can exhibit optical isomerism. However, not all ketones are capable of exhibiting this type of isomerism due to the absence of stereogenic carbon atoms or the presence of a symmetry plane. For example, consider the molecule 2,3-pentadione (CH3COCOCH3). In this ketone, the carbonyl group is bonded to two different stereogenic carbon atoms, so it can exhibit optical isomerism with two enantiomers. To sum up, ketones can exhibit two types of isomerism: structural isomerism (chain isomerism and positional isomerism) and stereoisomerism (optical isomerism). It is essential to consider the molecular formula, structural formula, and spatial arrangement when identifying the types of isomerism exhibited by ketones.